A design and assessment methods of a stand-alone system using a straight-wind vertical axis wind turbine are systematically developed for its optimal design. The target stand-alone system is mainly operated at a constant tip-speed ratio where the power coefficient has the maximum value and introduces a constant speed operation at high wind speeds. The specifications of the straight-wing vertical axis wind turbine based on given design parameters including solidity, aspect ratio, and blade airfoil are determined through a strength and power curve calculations employing an aerodynamic load analysis. Then, the generating performance and fatigue load characteristic of the system with the determined specifications are analyzed by using our dynamic simulation model. The numerical analysis shows that the specifications and performances of the system are influenced more by the solidity and aspect ratio than by the blade airfoil characteristic.